systemd is a suite of basic building blocks for a Linux system. It provides a system and service manager that runs as PID 1 and starts the rest of the system. systemd provides aggressive parallelization capabilities, uses socket and D-Bus activation for starting services, offers on-demand starting of daemons, keeps track of processes using Linux control groups, maintains mount and automount points, and implements an elaborate transactional dependency-based service control logic. systemd supports SysV and LSB init scripts and works as a replacement for sysvinit. Other parts include a logging daemon, utilities to control basic system configuration like the hostname, date, locale, maintain a list of logged-in users and running containers and virtual machines, system accounts, runtime directories and settings, and daemons to manage simple network configuration, network time synchronization, log forwarding, and name resolution.

Note: For a detailed explanation of why Arch moved to systemd, see this forum post.

Using units

When using systemctl, you generally have to specify the complete name of the unit file, including its suffix, for example sshd.socket. There are however a few short forms when specifying the unit in the following systemctl commands:

If you do not specify the suffix, systemctl will assume .service. For example, netctl and netctl.service are equivalent.

Mount points will automatically be translated into the appropriate .mount unit. For example, specifying /home is equivalent to home.mount.

Similar to mount points, devices are automatically translated into the appropriate .device unit, therefore specifying /dev/sda2 is equivalent to dev-sda2.device.

Note: Some unit names contain an @ sign (e.g. name@string.service): this means that they are instances of a template unit, whose actual file name does not contain the string part (e.g. name@.service). string is called the instance identifier, and is similar to an argument that is passed to the template unit when called with the systemctl command: in the unit file it will substitute the %i specifier.

To be more accurate, before trying to instantiate the name@.suffix template unit, systemd will actually look for a unit with the exact name@string.suffix file name, although by convention such a "clash" happens rarely, i.e. most unit files containing an @ sign are meant to be templates. Also, if a template unit is called without an instance identifier, it will just fail, since the %i specifier cannot be substituted.

Tip:

Most of the following commands also work if multiple units are specified, see systemctl(1) for more information.

The --now switch can be used in conjunction with enable, disable, and mask to respectively start, stop, or mask the unit immediately rather than after rebooting.

A package may offer units for different purposes. If you just installed a package, pacman -Qql package | grep -Fe .service -e .socket can be used to check and find them.

Start a unit immediately:

# systemctl start unit

Stop a unit immediately:

# systemctl stop unit

Restart a unit:

# systemctl restart unit

Ask a unit to reload its configuration:

# systemctl reload unit

Show the status of a unit, including whether it is running or not:

$ systemctl status unit

Check whether a unit is already enabled or not:

$ systemctl is-enabled unit

Enable a unit to be started on bootup:

# systemctl enable unit

Enable a unit to be started on bootup and Start immediately:

# systemctl enable --now unit

Disable a unit to not start during bootup:

# systemctl disable unit

Mask a unit to make it impossible to start it (both manually and as a dependency, which makes masking dangerous):

# systemctl mask unit

Unmask a unit:

# systemctl unmask unit

Show the manual page associated with a unit (this has to be supported by the unit file):

$ systemctl help unit

Reload systemd manager configuration, scanning for new or changed units:

Note: This does not ask the changed units to reload their own configurations. See reload example above.

# systemctl daemon-reload

Power management

polkit is necessary for power management as an unprivileged user. If you are in a local systemd-logind user session and no other session is active, the following commands will work without root privileges. If not (for example, because another user is logged into a tty), systemd will automatically ask you for the root password.

Shut down and reboot the system:

$ systemctl reboot

Shut down and power-off the system:

$ systemctl poweroff

Suspend the system:

$ systemctl suspend

Put the system into hibernation:

$ systemctl hibernate

Put the system into hybrid-sleep state (or suspend-to-both):

$ systemctl hybrid-sleep

Writing unit files

The syntax of systemd's unit files is inspired by XDG Desktop Entry Specification .desktop files, which are in turn inspired by Microsoft Windows .ini files. Unit files are loaded from multiple locations (to see the full list, run systemctl show --property=UnitPath), but the main ones are (listed from lowest to highest precedence):

/usr/lib/systemd/system/: units provided by installed packages

/etc/systemd/system/: units installed by the system administrator

Note:

The load paths are completely different when running systemd in user mode.

systemd unit names may only contain ASCII alphanumeric characters, underscores and periods. All other characters must be replaced by C-style "\x2d" escapes, or employ their predefined semantics ('@', '-'). See systemd.unit(5) and systemd-escape(1) for more information.

Tip: Comments prepended with # may be used in unit-files as well, but only in new lines. Do not use end-line comments after systemd parameters or the unit will fail to activate.

Handling dependencies

With systemd, dependencies can be resolved by designing the unit files correctly. The most typical case is that the unit A requires the unit B to be running before A is started. In that case add Requires=B and After=B to the [Unit] section of A. If the dependency is optional, add Wants=B and After=B instead. Note that Wants= and Requires= do not imply After=, meaning that if After= is not specified, the two units will be started in parallel.

Dependencies are typically placed on services and not on #Targets. For example, network.target is pulled in by whatever service configures your network interfaces, therefore ordering your custom unit after it is sufficient since network.target is started anyway.

Service types

There are several different start-up types to consider when writing a custom service file. This is set with the Type= parameter in the [Service] section:

Type=simple (default): systemd considers the service to be started up immediately. The process must not fork. Do not use this type if other services need to be ordered on this service, unless it is socket activated.

Type=forking: systemd considers the service started up once the process forks and the parent has exited. For classic daemons use this type unless you know that it is not necessary. You should specify PIDFile= as well so systemd can keep track of the main process.

Type=oneshot: this is useful for scripts that do a single job and then exit. You may want to set RemainAfterExit=yes as well so that systemd still considers the service as active after the process has exited.

Type=notify: identical to Type=simple, but with the stipulation that the daemon will send a signal to systemd when it is ready. The reference implementation for this notification is provided by libsystemd-daemon.so.

Type=dbus: the service is considered ready when the specified BusName appears on DBus's system bus.

Type=idle: systemd will delay execution of the service binary until all jobs are dispatched. Other than that behavior is very similar to Type=simple.

Editing provided units

This article or section needs language, wiki syntax or style improvements. See Help:Style for reference.

Reason: Should be renamed to more descriptive Modifying provided units. (Discuss in Talk:Edit#Deprecation)

To avoid conflicts with pacman, unit files provided by packages should not be directly edited. There are two safe ways to modify a unit without touching the original file: create a new unit file which overrides the original unit or create drop-in snippets which are applied on top of the original unit. For both methods, you must reload the unit afterwards to apply your changes. This can be done either by editing the unit with systemctl edit (which reloads the unit automatically) or by reloading all units with:

# systemctl daemon-reload

Tip:

You can use systemd-delta to see which unit files have been overridden or extended and what exactly has been changed.

Use systemctl cat unit to view the content of a unit file and all associated drop-in snippets.

Replacement unit files

To replace the unit file /usr/lib/systemd/system/unit, create the file /etc/systemd/system/unit and reenable the unit to update the symlinks:

# systemctl reenable unit

Alternatively, run:

# systemctl edit --full unit

This opens /etc/systemd/system/unit in your editor (copying the installed version if it does not exist yet) and automatically reloads it when you finish editing.

Note: The replacement units will keep on being used even if Pacman updates the original units in the future. This method makes system maintenance more difficult and therefore the next approach is preferred.

Drop-in files

To create drop-in files for the unit file /usr/lib/systemd/system/unit, create the directory /etc/systemd/system/unit.d/ and place .conf files there to override or add new options. systemd will parse and apply these files on top of the original unit.

The easiest way to do this is to run:

# systemctl edit unit

This opens the file /etc/systemd/system/unit.d/override.conf in your text editor (creating it if necessary) and automatically reloads the unit when you are done editing.

Note: Not all keys can be overridden with drop-in files. For example, for changing Conflicts= a replacement file is necessary.

Revert to vendor version

To revert any changes to a unit made using systemctl edit do:

# systemctl revert unit

Examples

For example, if you simply want to add an additional dependency to a unit, you may create the following file:

/etc/systemd/system/unit.d/customdependency.conf

[Unit]
Requires=new dependency
After=new dependency

As another example, in order to replace the ExecStart directive for a unit that is not of type oneshot, create the following file:

/etc/systemd/system/unit.d/customexec.conf

[Service]
ExecStart=
ExecStart=new command

Note how ExecStart must be cleared before being re-assigned [1]. The same holds for every item that can be specified multiple times, e.g. OnCalendar for timers.

One more example to automatically restart a service:

/etc/systemd/system/unit.d/restart.conf

[Service]
Restart=always
RestartSec=30

Targets

This article or section needs language, wiki syntax or style improvements. See Help:Style for reference.

systemd uses targets which serve a similar purpose as runlevels but act a little different. Each target is named instead of numbered and is intended to serve a specific purpose with the possibility of having multiple ones active at the same time. Some targets are implemented by inheriting all of the services of another target and adding additional services to it. There are systemdtargets that mimic the common SystemVinit runlevels so you can still switch targets using the familiar telinit RUNLEVEL command.

Get current targets

The following should be used under systemd instead of running runlevel:

$ systemctl list-units --type=target

Create custom target

The runlevels that held a defined meaning under sysvinit (i.e., 0, 1, 3, 5, and 6); have a 1:1 mapping with a specific systemdtarget. Unfortunately, there is no good way to do the same for the user-defined runlevels like 2 and 4. If you make use of those it is suggested that you make a new named systemdtarget as /etc/systemd/system/your target that takes one of the existing runlevels as a base (you can look at /usr/lib/systemd/system/graphical.target as an example), make a directory /etc/systemd/system/your target.wants, and then symlink the additional services from /usr/lib/systemd/system/ that you wish to enable.

Mapping between SysV runlevels and systemd targets

SysV Runlevel

systemd Target

Notes

0

runlevel0.target, poweroff.target

Halt the system.

1, s, single

runlevel1.target, rescue.target

Single user mode.

2, 4

runlevel2.target, runlevel4.target, multi-user.target

User-defined/Site-specific runlevels. By default, identical to 3.

3

runlevel3.target, multi-user.target

Multi-user, non-graphical. Users can usually login via multiple consoles or via the network.

5

runlevel5.target, graphical.target

Multi-user, graphical. Usually has all the services of runlevel 3 plus a graphical login.

6

runlevel6.target, reboot.target

Reboot

emergency

emergency.target

Emergency shell

Change current target

In systemd targets are exposed via target units. You can change them like this:

# systemctl isolate graphical.target

This will only change the current target, and has no effect on the next boot. This is equivalent to commands such as telinit 3 or telinit 5 in Sysvinit.

Change default target to boot into

The standard target is default.target, which is a symlink to graphical.target. This roughly corresponds to the old runlevel 5.

To verify the current target with systemctl:

$ systemctl get-default

To change the default target to boot into, change the default.target symlink. With systemctl:

systemd.unit=multi-user.target (which roughly corresponds to the old runlevel 3),

systemd.unit=rescue.target (which roughly corresponds to the old runlevel 1).

Default target order

Systemd chooses the default.target according to the following order:

Kernel parameter shown above

Symlink of /etc/systemd/system/default.target

Symlink of /usr/lib/systemd/system/default.target

Temporary files

"systemd-tmpfiles creates, deletes and cleans up volatile and temporary files and directories." It reads configuration files in /etc/tmpfiles.d/ and /usr/lib/tmpfiles.d/ to discover which actions to perform. Configuration files in the former directory take precedence over those in the latter directory.

Configuration files are usually provided together with service files, and they are named in the style of /usr/lib/tmpfiles.d/program.conf. For example, the Samba daemon expects the directory /run/samba to exist and to have the correct permissions. Therefore, the samba package ships with this configuration:

/usr/lib/tmpfiles.d/samba.conf

D /run/samba 0755 root root

Configuration files may also be used to write values into certain files on boot. For example, if you used /etc/rc.local to disable wakeup from USB devices with echo USBE > /proc/acpi/wakeup, you may use the following tmpfile instead:

Note: This method may not work to set options in /sys since the systemd-tmpfiles-setup service may run before the appropriate device modules is loaded. In this case you could check whether the module has a parameter for the option you want to set with modinfo module and set this option with a config file in /etc/modprobe.d. Otherwise you will have to write a udev rule to set the appropriate attribute as soon as the device appears.

Timers

A timer is a unit configuration file whose name ends with .timer and encodes information about a timer controlled and supervised by systemd, for timer-based activation. See systemd/Timers.

Mounting

systemd is in charge of mounting the partitions and filesystems specified in /etc/fstab. The systemd-fstab-generator(8) translates all the entries in /etc/fstab into systemd units, this is performed at boot time and whenever the configuration of the system manager is reloaded.

systemd extends the usual fstab capabilities and offers additional mount options. These affect the dependencies of the mount unit, they can for example ensure that a mount is performed only once the network is up or only once another partition is mounted. The full list of specific systemd mount options, typically prefixed with x-systemd., is detailed in systemd.mount(5).

An example of these mount options in the context of automounting, which means mounting only when the resource is required rather than automatically at boot time, is provided in fstab#Automount with systemd.

If using systemd-networkd, systemd-networkd-wait-online.service is by default enabled automatically whenever systemd-networkd.service has been enabled; check this is the case with systemctl is-enabled systemd-networkd-wait-online.service, there is no other action needed.

Enable installed units by default

This article or section needs expansion.

Reason: How does it work with instantiated units? (Discuss in Talk:Systemd#)

Arch Linux ships with /usr/lib/systemd/system-preset/99-default.preset containing disable *. This causes systemctl preset to disable all units by default, such that when a new package is installed, the user must manually enable the unit.

If this behavior is not desired, simply create a symlink from /etc/systemd/system-preset/99-default.preset to /dev/null in order to override the configuration file. This will cause systemctl preset to enable all units that get installed—regardless of unit type—unless specified in another file in one systemctl preset's configuration directories. User units are not affected. See systemd.preset(5) for more information.

Note: Enabling all units by default may cause problems with packages that contain two or more mutually exclusive units. systemctl preset is designed to be used by distributions and spins or system administrators. In the case where two conflicting units would be enabled, you should explicitly specify which one is to be disabled in a preset configuration file as specified in the manpage for systemd.preset.

The enhancement of an existing systemd unit file with application sandboxing typically requires trial-and-error tests accompanied by the generous use of strace, stderr and journalctl error logging and output facilities. You may want to first search upstream documentation for already done tests to base trials on.

Some examples on how sandboxing with systemd can be deployed:

CapabilityBoundingSet defines a whitelisted set of allowed capabilities, but may also be used to blacklist a specific capability for a unit.

The CAP_SYS_ADM capability, for example, which should be one of the goals of a secure sandbox: CapabilityBoundingSet=~ CAP_SYS_ADM

Troubleshooting

Investigating systemd errors

As an example, we will investigate an error with systemd-modules-load service:

1. Lets find the systemd services which fail to start at boot time:

$ systemctl --state=failed

systemd-modules-load.service loaded failed failed Load Kernel Modules

Another way is to live log systemd messages:

$ journalctl -fp err

2. Ok, we found a problem with systemd-modules-load service. We want to know more:

5. The Failed to find module 'blacklist usblp' error message might be related to a wrong setting inside of blacklist.conf. Lets deactivate it with inserting a trailing # before each option we found via step 3:

/etc/modules-load.d/blacklist.conf

# blacklist usblp
# install usblp /bin/false

6. Now, try to start systemd-modules-load:

# systemctl start systemd-modules-load

If it was successful, this should not prompt anything. If you see any error, go back to step 3 and use the new PID for solving the errors left.

If everything is ok, you can verify that the service was started successfully with:

Diagnosing a service

If some systemd service misbehaves or you want to get more information about what is happening, set the SYSTEMD_LOG_LEVELenvironment variable to debug. For example, to run the systemd-networkd daemon in debug mode:

then restartsystemd-networkd and watch the journal for the service with the -f/--follow option.

Shutdown/reboot takes terribly long

If the shutdown process takes a very long time (or seems to freeze) most likely a service not exiting is to blame. systemd waits some time for each service to exit before trying to kill it. To find out if you are affected, see this article.

Short lived processes do not seem to log any output

If journalctl -u foounit does not show any output for a short lived service, look at the PID instead. For example, if systemd-modules-load.service fails, and systemctl status systemd-modules-load shows that it ran as PID 123, then you might be able to see output in the journal for that PID, i.e. journalctl -b _PID=123. Metadata fields for the journal such as _SYSTEMD_UNIT and _COMM are collected asynchronously and rely on the /proc directory for the process existing. Fixing this requires fixing the kernel to provide this data via a socket connection, similar to SCM_CREDENTIALS. In short, it is a bug. Keep in mind that immediately failed services might not print anything to the journal as per design of systemd.

Boot time increasing over time

After using systemd-analyze a number of users have noticed that their boot time has increased significantly in comparison with what it used to be. After using systemd-analyze blameNetworkManager is being reported as taking an unusually large amount of time to start.

The problem for some users has been due to /var/log/journal becoming too large. This may have other impacts on performance, such as for systemctl status or journalctl. As such the solution is to remove every file within the folder (ideally making a backup of it somewhere, at least temporarily) and then setting a journal file size limit as described in Systemd/Journal#Journal size limit.

systemd-tmpfiles-setup.service fails to start at boot

Starting with systemd 219, /usr/lib/tmpfiles.d/systemd.conf specifies ACL attributes for directories under /var/log/journal and, therefore, requires ACL support to be enabled for the filesystem the journal resides on.

Disable emergency mode on remote machine

You may want to disable emergency mode on a remote machine, for example, a virtual machine hosted at Azure or Google Cloud. It is because if emergency mode is triggered, the machine will be blocked from connecting to network.